Field of the Disclosure
The present application relates to scanning electron microscopes, electron and X-ray detectors suitable for use in scanning electron microscopes, and systems and methods for reviewing and inspecting samples. The electron microscopes, detectors, systems and methods are particularly suitable for use in review and inspection systems including those used to review and/or inspect photomasks, reticles, and semiconductor wafers.
Related Art
The integrated circuit industry requires inspection tools with increasingly higher sensitivity to detect ever smaller defects and particles whose sizes may be a few tens of nanometers (nm), or less. These inspection tools must operate at high speed in order to inspect a large fraction, or even 100%, of the area of photomask, reticle or wafer, in a short period of time, e.g. one hour or less for inspection during production, or, at most, few hours for R&D or troubleshooting. In order to inspect so quickly, inspection tools use pixel or spot sizes larger than the dimensions of the defect or particle of interest, and detect just a small change in signal caused by a defect or particle. High speed inspection is most commonly performed in production using inspection tools operating with UV light. Inspection in R&D may be performed with UV light or with electrons.
Once a defect or particle has been found by high speed inspection, it is often necessary to make a higher resolution image and/or to perform material analysis to determine the origin or type of the particle or defect. This process is commonly called review. Review is usually performed with a scanning electron microscope (SEM). Review SEMs used in the semiconductor manufacturing process are typically required to review many thousands of potential defects or particles per day and so may have, at most, a few seconds per target for review. Review SEMs for the semiconductor and related industries are manufactured by KLA-Tencor Corporation (for example the eDR-7110), by Applied Materials, Inc. (for example the SEMVision G6) and other companies.
A review SEM most commonly detects secondary electrons emitted from the sample in order to form an image. An exemplary secondary electron detector for a review SEM is described in U.S. Pat. No. 7,141,791 to Masnaghetti et al. entitled “Apparatus and method for e-beam dark-field imaging”. This exemplary secondary electron detector includes electron optics for collecting secondary electrons and directing them to a scintillator. The electrons are accelerated towards the scintillator so that each electron striking the scintillator causes multiple photons to be emitted. Some of those photons are captured by a light pipe and directed to one or more photomultiplier tubes. A disadvantage of this approach is that the detector is relatively slow. The light emission from scintillator has a decay time constant of several, or many, tens of ns. Furthermore scintillators have multiple time constants. The initial response may have a time constant of a few tens of ns, but light emission will continue at a low level with a much longer time constant. Photomultiplier tubes also have responses with multiple time constants. The photocathode that emits photoelectrons has one, or more, time constants. The electrons take a significant time to travel from one dynode to another and finally to the anode, which creates an additional time constant. The electron travel time can be reduced by reducing the number of dynodes, but this reduces the gain of the photomultiplier tube and so is not a desirable trade-off as it reduces the sensitivity of the SEM in order to improve the speed.
SEMs designed for review may include electron microprobe (X-ray) analysis for material identification. In order for an SEM to have image resolution of a few nm or better to provide high-quality images of nm-sized defects and particles, the sample being inspected is placed close to the final objective lens so that it is immersed in the magnetic field of that lens, thus minimizing imaging aberrations. Placing the sample close to the objective lens prevents large detectors being placed close to the sample. In particular, X-ray detectors used for microprobe or similar analysis can only collect X-rays in a small solid angle, making such systems very slow. Many tens of seconds or minutes of data acquisition time may be required per target to capture enough X-rays to determine the material composition of the target.
The final objective lens of a review SEM also limits where secondary and backscattered electron detectors may be placed and the collection voltages that can be applied to those detectors. A small potential difference (such as less than about 2 kV) between an electron detector and the sample reduces the efficiency and sensitivity of an electron detector to collect and detect low-energy electrons from the sample.
Therefore, a need arises for a high-speed high-resolution review SEM overcoming some, or all, of the above disadvantages. In particular a need arises for a high-speed high-resolution automated SEM with capability to quickly identify at least some commonly used materials. It is further desirable that capability to quickly identify materials and/or provide improved image contrast be included in a high-speed inspection SEM.